Mercury’s finest hour

You can do something the Hubble telescope can't: Observe Mercury. And February is the best time this year to do it.

The first planet will make its best appearance of 2013 about 30 minutes after sunset between Feb. 12 and 20 as it positions itself at least 10 degrees-extend your arm and make a fist-above the horizon in the west-southwest. It may be tough to view since we don't live on a prairie and trees obstruct most of the horizons in our backyards.

Mercury and Venus are the only two planets between Earth and the sun. Because of this vantage point, Mercury appears to have phases like the moon-first quarter, last quarter and half-illuminated.

Since its orbit hugs the sun so tightly, Mercury is only visible from Earth during morning or evening twilight, always rising and setting with the sun. Hubble as a rule won't observe it because of its proximity to the sun. Training its powerful telescope on Mercury (or Venus, for that matter) would likely damage Hubble's instruments.

Mercury will be in retrograde from Feb. 8 into April, meaning it will appear to move backward in the sky, but this is an optical illusion caused by the difference in orbital speeds of the two planets. Mercury periodically reaches a stage in its orbit around the sun where, from our perspective on Earth, it appears to slow down to the point where it stands still, and then appears to move backward.

Mercury will be less than 1 degree from Mars on Feb. 8. The sky won't be dark enough to spot the pair until they're just setting in the west.

Since the new moon is on Feb. 10, just after sunset on Feb. 11, a very thin crescent moon will be setting right behind the sun and floating above Mercury and Mars. After the weekend of Feb. 16 and 17, Mercury begins to slide back toward the horizon and gets lost in the sun's glare, ending our Earthly observing opportunities for now.

SKY CALENDAR

Feb. 7 - Mercury and Mars form a pleasing but tight planetary conjunction. Mars will be two-thirds of a degree to the upper left of much brighter Mercury. Binoculars or a telescope are recommended because they will be very low in the west-southwest at evening twilight.

Feb. 10 - New moon.

Feb. 25 - Full moon. This full moon was known by early Native American tribes as the Full Snow Moon because the heaviest snows usually fell during this time of the year. Since hunting is difficult in late February, this moon has also been called the Full Hunger Moon.